This application claims the priority of Germany patent Document 199 11 018.2, filed Mar. 12, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an auxiliary power unit for aircraft, abbreviated APU.
It is an object of the auxiliary power unit on board aircraft to generate compressed air for air conditioning the cabin and starting the power unit and to generate electric energy for the on-board power supply. The auxiliary power unit will be used mainly when the aircraft is on the ground and permits a supply of the aircraft independently of external sources. Furthermore, the auxiliary power unit is partially used as a safety system in the case of an engine failure for relieving the remaining main engines during the flight.
Systems which are currently in use have the construction schematically illustrated in FIG. 1. The illustrated auxiliary power unit comprises a gas turbine GT which, as essential components, has a compressor V, a combustion chamber BK as well as a turbine T. By way of a common shaft, the turbine T drives a compressor K for generating compressed air (for example, 3 bar for supplying the cabin air-conditioning system). A generator GN for generating electric energy (for example, 380 V/400 Hz for the on-board power supply) is also coupled with the turbine T. By way of a transmission, additional auxiliary power units can be driven by means of the gas turbine GT.
The disadvantage of the known auxiliary power units is their relatively poor efficiency, particularly with respect to generating electricity. Their operation is also connected with high exhaust gas and noise emissions.
Furthermore, in vehicle construction, the use of fuel cells is known for driving purposes as well as for supplying electric energy.
It is an object of the invention to provide an auxiliary power unit by means of which electricity can be generated on board at a high efficiency and while reducing emissions.
This task is achieved by providing an auxiliary power unit of the above described general type, characterized in that a fuel cell is present for generating electric energy.
According to the invention, the electricity generating part of the conventional auxiliary power unit is replaced by a fuel cell system.
In the case of conventionally powered aircraft, the hydrogen required by the fuel cell can be generated by a suitable hydrogen generating unit, particularly a reformer stage, from the aviation gasoline/kerosine present on board. As an alternative, the hydrogen for the operation of the auxiliary power unit could also be carried along in a tank.
In the case of a hydrogen plane, in which the fuel carried along for the propulsion of the aircraft is liquid hydrogen, the fuel cell can be supplied directly with the hydrogen present on board.
Therefore, on the whole, in the case of conventional airplanes as well as in the case of hydrogen airplanes, the electric energy can be generated from the fuel for the propulsion of the aircraft which is present on board anyhow. As an alternative, an external supply on the ground is also conceivable.
For the air supply to the fuel cell, particularly the following embodiments can be used:
In a preferred embodiment, the compressing of the fuel cell process air takes place by means of the compressor which is coupled with the turbine and which also generates the compressed air for the on-board air-conditioning system.
In another embodiment, the fuel cell is supplied with air by way of the compressor of the gas turbine.
As an alternative, one or several additional compressor stages can be present on the turbine shaft, specifically for the compressing of the fuel cell process air.
The air supply to the fuel cell can also be achieved by a cooling ventilator present in the auxiliary power unit for cooling purposes. In this embodiment, the fed process air has a relatively low pressure level.
If a pneumatic on-board air-conditioning system with an expansion turbine exists on board, this expansion turbine can be used for feeding and compressing the process air.
In order to utilize the energy content of the fuel cell waste gases, the anode waste gas as well as the cathode waste gas can be introduced into the combustion chamber of the gas turbine. Typically, only a fraction of approximately 80% of the fed hydrogen can be utilized in the fuel cell. The energy content of the waste gas is sufficient for covering approximately 30 to 50% of the compression energy to be applied for the fuel supply to the fuel cell.
Particularly, a PEM fuel cell (Proton Exchange Membrane fuel cell) can be used as the fuel cell.
Summarizing, the following advantages of the auxiliary power unit according to the invention are obtained:
Generating electricity at a high efficiency and from fuel which already exists on board the aircraft;
reduction of the fuel consumption as well as of the exhaust gas and noise emissions;
extensive uncoupling of the generating of electricity and the air compression;
increase of the efficiency particularly also in the partial load operation;
no separate compressor is required for the air supply to the fuel cell; on the contrary, the devices for the air compression existing on board can be used.